Our copending application Ser. No. 812,757 teaches an improved method of making nodular iron castings by treating molten iron in the runner system of a mold with a mixture of magnesium metal particles and iron diluent particles. Contrary to prior art teachings, we descovered that unalloyed magnesium metal may be used successfully in the mold, without excessive fuming or pyrotechnics, when the additive comprises a homogeneous mechanical blend of magnesium and iron diluent particles. All the particles are substantially in the size range of from about 0.15 to 5 mm, and the weight ratio of magnesium to iron is in the range of from about 1:12 to 1:20. Use of our mixtures insures more consistent treatment from mold to mold, and less magnesium waste due to magnesium loss in producing prealloyed magnesium-ferrosilicon alloy inoculants.
Our method of treating iron in the mold with magnesium metal was developed using treatment-size particle batches of a few kilograms each. In the foundry it would be inconvenient to mix up a separate small batch of nodularizing additive for each mold. It is much more desirable to provide the foundryman with our particulate additives in premixed bulk quantities from which small portions may be withdrawn for in-the-mold treatments. However, all mixtures of magnesium and iron diluent particles in the 0.15 to 5.00 mm range are not stable during transportation and storage. In some, the smaller particles segragate toward the bottom of a container, and the magnesium particles become unevenly dispersed throughout the mixture. Neither of these situations is tolerable where the success of the iron treatment method depends on the predictability of the magnesium content of a given weight of additive.
Thus, it is an object of this invention to provide large batches of homogeneously mixed magnesium metal and iron diluent particles from which smaller portions may be randomly withdrawn to predictably and consistently inoculate molten iron. It is a more specific object to provide the foundryman with bulk premixed batches of such particles which batches are transportation and storage stable in drums or other large containers so that small amounts can be taken therefrom to treat molten iron in the runner of a mold and form nodular iron castings. It is another object to provide unalloyed magnesium-iron diluent particle mixtures such that the overall size distribution of particles throughout is substantially uniform and the relative magnesium to diluent content of random samples of the mixture is also substantially constant. It is yet another object to specify, before mixing, which stocks of magnesium and iron diluent particles can be combined to form our transportation and storage stable mixtures. It is a further object to specify allowable relative size ranges between the magnesium and the iron diluent particles for such mixtures.